Positron Emission Tomography (PET) is a medical imaging technique where radioactive 'tracer' isotopes which emit a positron are injected into a living subject. After travelling about 1 millimeter the positron annihilates with an electron, producing a pair of gamma rays. By measuring where the gamma rays end up, their origin in the body can be plotted, allowing the chemical uptake or activity of certain part of the body to be determined. It is used heavily in studies of the human brain.

PET as a technique for scientific investigation is limited by the need for clearance by ethics committees to inject radioactive material into subjects, and also by the fact that it is not advisable to subject any one subject to too many scans. Furthermore PET isotopes have a very short half-life, making shipment difficult or impossible, and few hospitals or universities own the expensive cyclotron required to produce them.

PET scanning of the brain is also based on an assumption that areas of high radioactivity are associated with brain activity, and this is a reasonable assumption. What is actually measured indirectly is the flow of blood to different parts of the brain, which is generally believed to be correlated, and usually measured by detecting radioactive oxygen.

PET scanning is a very valuable technique for some diseases and disorders, because it is possible to target the radio-chemicals used for particular bodily functions.

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